In various factories and plants, for the purpose of information exchange and an emergency call between an administration department and a field, for example, a high frequency wireless communication system in a high frequency band of 1.9 GHz has been used.
FIG. 5 shows an example of such a high frequency wireless communication system. The high frequency wireless communication system includes plural fixed wireless devices (wireless base stations) 3a and 3b connected to a private branch exchange 1 by communication lines 2a and 2b. In addition, in the high frequency wireless communication system, through the fixed wireless devices 3a and 3b and antennas 4a and 4b, wireless communication is performed between plural mobile terminals 5a, 5b and . . . in an area and the private branch exchange 1. That is, when such a high frequency wireless communication system is used, through the fixed wireless device 3a and 3b, a phone call can be made between another telephone 6 connected to the private branch exchange 1 and the mobile terminals 5a and 5b in the field, and an emergency notification can be concurrently transmitted to each mobile terminal 5a to 5f from the administration department through each fixed wireless device 3a and 3b. 
However, when the above-mentioned high frequency wireless communication system is introduced into an oil plant and a gas fuel power plant handling volatile gas, each fixed wireless device 3a and 3b provided in an explosion-proof region is demanded to have an explosion-proof structure to prevent an explosion accident before happens.
FIG. 6 is a view showing a pressure-resistant explosion-proof container of the related art which is formed to have a pressure-resistant explosion-proof structure. In FIG. 6, an antenna attachment hole 21 is provided on a peripheral surface of an explosion-proof device main body 22.
A 45° elbow-type joint 23 is attached at one end to the antenna attachment hole 21 through an O ring 23a while satisfying the pressure-resistant explosion-proof structure conditions of a joint surface.
That is, the 45° elbow-type joint 23 is screwed into the explosion-proof device main body 22, and a screw specification is a structure having pressure-resistant explosion-proof performance.
In the 45° elbow-type joint 23, an antenna position fixing lock nut 23b is attached to the antenna attachment hole 21.
By loosening the antenna position fixing lock nut 23b to rotate the 45° elbow-type joint 23, the explosion-proof device main body 22 can be installed such that the antenna direction is aligned to a polarization plane even when the installation position of the explosion-proof device main body 22, for example, the explosion-proof device main body 22 is changed from a horizontal position to a vertical position.
The horizontal position, vertical position and polarization plane of the antenna can be aligned by rotating the antenna by 180 degrees.
An antenna cover 24 is attached at one end to the other end of the 45° elbow-type joint 23 through an O ring 24a while satisfying the pressure-resistant explosion-proof structure conditions of the joint surface, and has an antenna 25 therein while satisfying the strength conditions of the pressure-resistant explosion-proof structure.
That is, the antenna has a minute gap and a sufficient length of fit between the antenna cover 24 and the 45° elbow-type joint 23, and has a structure satisfying the pressure-resistant explosion-proof standard.
The antenna cover 24 and the 45° elbow-type joint 23 are fixed by an antenna cover fixing lock nut 24b. 
In the configuration of FIG. 6, a circuit and a high frequency connector that is an antenna connection unit have a structure that resists pressure, and a metal container and the connector have a pressure-resistant explosion-proof structure as a whole.
Then, a transmission high frequency signal is transmitted from the antenna through a connector unit as a high frequency signal, and a reception high frequency signal received by the antenna is transmitted to the circuit (not shown) through the connector unit.
FIGS. 7A and 7B are cross-sectional views showing other examples of the related art.
In FIG. 7A, an antenna 41 is disposed in a pressure-resistant explosion-proof container 40 formed of a robust metal. A part of the pressure-resistant explosion-proof container 40 is sealed by a glass window (or resin or like) 42 though which a high frequency signal passes. The antenna 41 is disposed around the glass window 42, and transmits and receives the high frequency signal through the glass window 42.
Since the high frequency signal does not pass through metal, a part of the container is necessary to be formed of glass or resin to install the antenna inside the container. In addition, in order to effectively receive and transmit the high frequency signal, it is necessary to increase the size of the window portion. That is, the high frequency signal is remarkably attenuated in an opening which is equal to or less than a specific size determined by a wavelength.
FIG. 7B is a view showing an example in which a glass window (or resin or the like) 42a is formed in a dome shape to widen antenna directivity.